Commonly, in a transmission projection screen, a Fresnel lens sheet and a lenticular sheet are assembled.
FIG. 15 is a general view for a television having conventional three-tube projector. In FIG. 15, reference numerals 51, 52, and 53 indicate image projection devices for colors such as R, G, and B. Reference numeral 54 indicates a mirror. Reference numeral 55 indicates a transmission projection screen.
A Fresnel lens sheet in FIG. 16 emits a light (which disperses from a small-diameter lens) which is projected from a projector to a lenticular sheet 57 as an approximate parallel light by using a converging lens.
The lenticular sheet 57 widens horizontally a range of the projected light which is incident as an approximate parallel light by the Fresnel lens sheet 56 so as to emit to a person who observes the light by using a characterized feature of a cylindrical lens group which are disposed in horizontal direction.
Also, in a rear-projection screen, customarily, a light diffusing layer is formed for purposes of widening a display light in a vertical direction, making the projected light from the projector focused, and reducing unnecessary blinking which is called a scintillation in the image because of small diameter of the lens in the projector.
The light diffusing layer is formed in at least any one among a lenticular sheet, Fresnel lens sheet, or a front board such as a protection board. For forming the light diffusing layer, methods such as applying methods, layering methods, and mixing methods can be employed preferably.
For a rear projection TV, according to a method which is used in a projector (light source), there are variations such as CRT method according to the three-tube (R, G, B) method, liquid crystal method, reflection light bulb method which is otherwise called as a digital-micromirror-device (DMD, Trademark registered by Texas Instruments (TI) or digital-light-processing (DLP). Liquid crystal method and reflection light bulb method are in so-called a single-tube method in which there is one set of projector.
In a conventional lenticular sheet which is used in a rear projection screen which is used for a rear projection TV according to CRT method, a lens section on a front surface and on a back surface are half-column-convex-shaped-cylindrical-lens-group in a uniform disposition direction, the disposition pitch of the unit lenses on a front surface and a back surface is 1:1 under accurate alignment condition (not shown in the drawings).
The above structure is employed because the projected light from each projector is emitted under offset angle condition each other when three projectors for three colors such as R, G, and B are disposed in a horizontal direction.
When the projected lights for R, G, and B are emitted from the projectors under offset angle condition, images according to three primary colors are mixed without position shift. Thus, observable angle is limited. When viewpoint is moved horizontally, image looks somewhat in red or in blue. Such observation accompanied with changed color is called “color shift”.
In order to solve the color shift, it is necessary to adapt a dual-surface lenticular sheet in which half-column-cylindrical-lens-group are formed on a front surface and a back surface, the disposition pitch of the unit lens on the front surface and the back surface is in relationship of 1:1, unit lenses which are to be a pair on the front surface and the back surface are accurately aligned.
Also, the shape of the unit lenses on the front surface and the back surface is designed by taking the refractive index corresponding to the wavelength such that the scope of the image for primary colors such as R, G, and B of which optical paths are slightly offset overlapping in a uniform optical intensity should be as large as possible.
In order to provide a high resolution image quality, in the lens sheet contained in the screen, narrow pitch (fine pitch) disposition in the unit lens is required. In order to produce a dual-surface-lenticular-sheet, alignment in the unit lens on the front surface and the back surface is more difficult.
Also, along with realization in finer resolution in parallel disposition pitch for a unit lens, finer resolution for a light transmitting section in a shading pattern (Black Matrix hereinafter called BM) which is formed away from the lens section has been realized. Therefore, accuracy for forming an aperture section in a light condensing section where a light is condensed by the micro-lens array section clearly is required.
In case of a lens sheet having a fine pitch lens section, a black matrix is formed by so called a self-alignment method in which positions of non-light-condensing section for the lens are determined accurately by using light condensing characteristics by the lens to a photosensitive resin layer which is formed on the lens sheet away from the lens.
For self alignment method, there are a wet method in which the shading pattern is formed after developing the exposed photosensitive resin layer, or a dry method in which the shading pattern is formed applying a color without developing the exposed photosensitive resin layer.
In dry method, a photosensitive adhesion agent having a characteristics in which bonding characteristics occurs according to whether or not light is exposed is used, and a color is applied corresponding to the adhesion.
In order to form a BM having preferable shading ratio (for obtaining preferable contrast in image, a range such as more than 60% is experimentally preferable) for a transmission liquid crystal projection screen, the position of light condensation (focus) by the lens section is set preferably according to the shading ratio in the formed pattern in the photosensitive resin layer not on the light emission of the photosensitive resin layer.
In a case in which a lens is spherical, positions of focal point are different between in the center of the lens and marginal area of the lens according to aberration. Therefore, in a case in which the shading pattern is formed by the self alignment method, the position where the characteristics of the photosensitive resin layer changes cannot be determined precisely. Therefore, a borderline between the aperture section and the shading section is hardly clear.
In particular, in a case in which the shading ratio is enhanced so as to realize an image with higher contrast, there is a problem in that it is difficult to form a BM having a clear borderline between a fine aperture section and the shading section when parallel disposition pitch between the unit lens is very fine and each one of the unit lens is fine.
In a conventional light transmission screen which is used in a rear-projection television which is sold in a market, it is common that a Fresnel lens having a concentric gaps are formed on one surface and a lenticular lens in which cylindrical lenses are disposed in one direction are provided. A case in which either one of them is used is acceptable. Also a case in which a light diffusing layer is provided on other member can be common.
In members which is contained in these transmission screens, optical disposition is arranged such that the lights which are emitted from the projector are in approximate parallel state by the Fresnel lens so as to obtain horizontal perspective angle by widening the emitted light in a horizontal direction in the image by the lenticular lens and vertical perspective angle by widening the emitted light in a vertical direction in the image by the light diffusing member.
Furthermore, a transmission screen which can obtain a brighter and clearer image quality is known by replacing the lenticular lens by a micro-lens sheet which can obtain the horizontal perspective angle and the vertical perspective angle compatibly without using the light diffusing member and by discontinuing or reducing the use of the light diffusing member.
Also, a method in which two lenticular layers are used such that longitudinal directions of the cylindrical lenses are orthogonal, or a method in which the cylindrical lenses are disposed on both surfaces of one base member layer such that the longitudinal directions of the cylindrical lenses are orthogonal are known.
Also, a structure in which a shading layer having aperture sections is provided in a light condensing section of each of the cylindrical lens contained in the lenticular lens or in the light condensing section of each of the micro-lens contained in the micro-lens array sheet so as to improve S/N ratio of the screen is known.
Also, there is a case in which a hardcoat layer or a anti-reflection layer (hereinafter called AR layer) are provided an outermost surface according to a usage of these transmission screen.
In a transmission screen using a conventional lenticular sheet or a micro-lens sheet, a lenticular layer and a diffusion layer are combined so as to control the perspective angle in a horizontal direction and a vertical direction. In the micro-lens array, wider perspective angle is necessary; therefore, there are disadvantages such as deteriorated quality in the image due to absorption of light by the diffusion layer and white scattering, and reduced screen gain caused by wide diffusion.
Also, it is possible to propose to control the perspective angle in a horizontal direction and a vertical direction by using two lenticular layer in which longitudinal direction of each cylindrical lenses are orthogonal, or by disposing a plurality of cylindrical lens on both surfaces of one base member layer such that longitudinal directions of each of the cylindrical lens are orthogonal. In such cases, quantity of the members contained in the cylindrical lens becomes doubled, and very fine manufacturing process for lens also becomes doubled. Thus, there is a problem in that cost for parts and manufacturing process become expensive.
Also, in a screen in which two lenticular lenses are disposed on one plane surface so as to be orthogonal each other, two lenticular lenses overlap each other. Therefore, if shape of one lenticular lens changes, optical characteristics in the other lenticular lens changes accordingly. Thus, it is not possible to control the perspective angle by independently changing the shape of one of the lenticular lenses. Therefore, there is a limit for controlling range for the perspective angle; thus, such a limit is not preferable from practical point of view.
Furthermore, in order to use a micro-lens array for a transmission screen, it is necessary to produce in a worthwhile size for the purpose such as 50 inches in diagonal plane. In such a case, a lens thug (thickness of refractive surface) having nearly half a diameter of an element lens is necessary for obtaining wider perspective angle. However, it is difficult to compatibly realize such depth in lens thug and picture size because of the depth to be molded. Because of this, even if necessary optical performance can be realized as long as an image area is small, due to a problem in manufacturing process, it is difficult to enlarge the image area.